U.S. President Donald Trump's administration certified on July 17 that Iran is complying with the 2015 nuclear deal and will continue to receive nuclear-related sanctions relief, but vowed to press ahead with sanctions over ballistic missiles, fast boats, and other matters.

It was the second time since Trump took office in January that the administration has certified Iranian compliance with the agreement, which Trump promised to "rip up" and called the "worst deal ever" during the election campaign in 2016.

While certifying that Tehran has adhered to the terms of the agreement it forged with Washington and five other world powers, the administration lambasted Iran for breaching what it called "the spirit" of the deal by continuing to develop and test ballistic missiles and fast boats -- matters it said would be targeted with further sanctions.

The administration also continued to express concern about Iran's alleged human rights abuses, support for terrorism against Israel, imprisonment of U.S. citizens, and interference with freedom of navigation in the Persian Gulf.

"Iran remains one of the most dangerous threats to U.S. interests and regional stability," a senior U.S. official told reporters on condition of anonymity, adding that the administration is looking at ways to try to strengthen the deal -- which placed curbs on Iran's nuclear program in an effort to ensure it does acquire atomic weapons -- and more strictly enforce it.

The late-evening announcement came hours after Tehran's top diplomat suggested that Iran has been struggling to understand the U.S. stance on Iran since Trump took office.

Iranian Foreign Minister Mohammad Javad Zarif (file photo)

"We receive contradictory signals, so we don't know how to interpret it," Iranian Foreign Minister Mohammad Javad Zarif told the Council on Foreign Relations in New York.

Zarif said that while the United States may be ambivalent, "Iran is serious about the nuclear deal" and will continue to comply with its restrictions on nuclear activities in exchange for the lifting of nuclear-related sanctions.

Zarif said he has not as yet spoken with Secretary of State Rex Tillerson, who is conducting a comprehensive review of U.S. policy toward Iran, but he is open to doing so.

"There are no communications between myself and Secretary Tillerson," he said. "It doesn't mean there can't be. The possibilities for engagement...have always been open."

By contrast, Zarif said during his years of negotiating the 2015 nuclear deal, he and former Secretary of State John Kerry "probably" spent more time with each other "than with anybody else."

Representatives of nuclear powers China, Russia, France, Britain, and the United States -- plus Germany -- are to meet in Vienna on July 21 to take stock of the deal.

Zarif said they would discuss Iran's claim that the United States has not been complying with the deal by failing to lift all sanctions against Iran. In fact, the deal does not require the United States to lift sanctions that are not related to Iran's nuclear activities.

But Zarif said that the Trump administration's moves to ratchet up nonnuclear sanctions on Iran -- along with legislation that recently passed the U.S. Senate to increase nonnuclear sanctions on Iran -- "creates the impression in Iran that the United States' hostility toward Iran will never end."

The United States says it maintains sanctions on Iran because the ballistic missiles Iran is testing have the capacity to carry nuclear warheads and can reach Israel, Tehran's foe. Iran denies that.

Zarif insisted that Iran's missiles are only intended for self-defense and he sarcastically compared Iran's homemade missiles with weapons that the United States is to supply Tehran's main rival in the Middle East, Saudi Arabia, under a recently announced $110 billion arms deal.

Noting that Iran fought an eight-year war against Iraq in the 1980s during which it endured chemical weapons attacks, Zarif said that "we need [the missiles] to make sure that another Saddam Hussein around the corner will not come and hit us again."